I am Windows-based, which seems to be a Bad Thing for RMagick installation. I got it working - briefly - then it stopped. In that time I managed to try gruff, which worked nicely enough. But the library is too much hassle for my short temper!

Protovis composes custom views of data with simple marks such as bars and dots. Unlike low-level graphics libraries that quickly become tedious for visualization, Protovis defines marks through dynamic properties that encode data, allowing inheritance, scales and layouts to simplify construction.
As Protovis, Rubyvis is free and open-source, provided under the BSD License. Its delivers SVG natively, which could be exported to PNG, JPEG and others graphics formats

rgplot is a ruby gnuplot gem. It enables gnuplot visualizations from ruby. I am a longtime fan of gnuplot, so I am biased - gnuplot gives a simple scripting approach to creating graphs. I also found something called rmagick which looks neat.